1. Field of the Invention
The present invention relates to a piston type compressor for use in, for example, an air conditioning system for a vehicle, and, more particularly, to a coupling structure for a cylinder block, a front housing and a rear housing.
2. Description of the Related Art
Generally, piston type compressors for use in a vehicular air conditioning system or a freezer are classified into a double-headed piston type and a single-headed piston type. The double-headed piston type compressor has a front housing connected to the front side of a cylinder block having a plurality of cylinder bores and a rear housing connected to the rear side thereof. As a drive shaft supported on the cylinder block rotates, the individual double-headed pistons reciprocate in the associated cylinder bores via a swash plate. Consequently, suction, compression and discharge of gas occurs in pairs of compression chambers located between the front and rear head portion of each double-headed piston and the associated front and rear housings.
The single-headed piston type compressor has a front housing connected to the front end surface of a cylinder block having a plurality of cylinder bores and a rear housing connected to the rear end surface thereof. The cylinder block and the front and rear housings are securely fastened with bolts. A drive shaft is supported on the front housing and the cylinder block. As the drive shaft rotates, the individual pistons reciprocate in the associated cylinder bores via a swash plate. As a result, gas suction, compression and discharge are executed in compression chambers located between the head portion of each piston and the rear housing.
In the single-headed piston type compressor, as shown in FIG. 12, a front housing 72 is connected to the front end surface of a cylinder block 71, and a rear housing 73 is connected to the rear end surface. Those components are securely fastened with through bolts 74, which penetrate through insertion holes 72a of the front housing 72, a crank chamber 75 and insertion holes 71a of the cylinder block 71 to engage the rear housing 73. Formed in the cylinder block 71 are a plurality of cylinder bores 71b for accommodating a plurality of single-headed pistons and a valve chamber 71c for accommodating a rotary valve for drawing in a refrigerant gas as shown in FIG. 13.
Because of the configuration of the housing and the through bolts 74 in this conventional compressor, the aluminum alloy cylinder block 71 flexes slightly and deforms resiliently due to the bending moment produced by the compressive force of the through bolts 74. As shown in FIG. 14, a mating surface between the cylinder block 71 and the front housing 72 forms a vertical surface 76 perpendicular to the center axis O of each through bolt 74. Therefore, fastening force f.sub.1 parallel to the center axis O acts on the front end surface of the cylinder block 71 from the front housing 72. Fastening force f.sub.2 acts on the rear end surface of the cylinder block 71 along the axis O from the rear housing 73.
Thus, the bending moment M acts around the center Po of a straight line H connecting the point of application P1 of the force f.sub.1 on the vertical surface 76 and the point of application P2 of the force f.sub.2 on the rear end surface of the cylinder block 71. This moment M is obtained by the following approximation equations: EQU fD.sub.2 =f.sub.2 D.sub.1 EQU f=(D.sub.1 f.sub.2)/D.sub.2 ( 1) EQU M=2f(D.sub.2 /2)=fD.sub.2 ( 2)
where D.sub.1 is the distance between both points of application P1 and P2 in the radial direction, D.sub.2 is the axial length of the cylinder block 71, and f is the radial component of the forces at the points of application P1 and P2.
The moment M obtained by the equations (1) and (2) acting on the cylinder block 71 causes slight resilient deformation of the shape of the cylinder block 71 as indicated with exaggeration by a two-dot chain line in FIG. 13. Over a period of time, such repeated resilient deformation can deform the cylinder bores 71b, which would then interfere with the smooth reciprocation of the pistons. When the valve chamber 71c for the rotary valve is formed as shown in FIG. 13, the thickness of the wall between each cylinder bore 71b and the valve chamber 71c is comparatively thinner and the rigidity of the wall is lower. Therefore, the cylinder bores 71b flex more easily during compressor operation. Ultimately, the inner wall of the valve chamber 71c may wear in the vicinity of the outer surface of the rotary valve, which would increase the overall frictional resistance as the rotary valve rotates. This tends to interfere with the smooth rotation of the rotary valve. If the through bolts 74 are provided in the cylinder block 71 and the front housing 72 outside the crank chamber 75 to permit the fastening forces f.sub.1 and f.sub.2 to act on the axial line O of each through bolt 74, no bending moment to deform the cylinder block 71 is produced. In this case, however, the outside diameter of the housing must be made larger. Such a compressor costs more and takes up more valuable space in the engine compartment.